For purifying air both for air conditioning and ventilation purposes, for instance in buildings or vehicles and also for vehicles driven by IC engines, as for instance gas turbines or diesel and gasoline engine for motor vehicles or all types it is customary to employ air filtering media and air filters produced therefrom, which possess a certain, limited capacity to hold dust and after reaching a certain limit, which may be expressed in terms of hours of use, differential pressure or mileage, must be replaced. The dust holding capacity is consequently measured in accordance with the maximum amount of dust, which the air filter is able to accept before a lower limit for a certain quantity of air passing through it, and consequently the end of its service life, is reached.
As a rule in the case of such applications the necessary level of the degree of separation is known or set, for example in terms of the filter class in air conditioning and ventilating technology or of a predetermined percentage of the particles to be separated dependent on the requirement and the test method for the respective internal combustion engine (for example the degrees of fractions separated or gravimetric degree of separation). Testing is performed in accordance with the method using a test aerosol (normally test dust, for example SAE fine or SAE coarse) under exactly set conditions such as impact flow speed, raw gas concentration, time and duration of the measurement, criterion for termination of measurement or the like. The degree of separation accordingly indicates what percentage of the dust is retained in the filter, and is substantially dependent of the pore size of the air filter.
The aim is to achieve maximum dust storage capacity and accordingly service life under these conditions. Since however the degree of separation on the one hand and the service life on the other hand correlate negatively with each other, it is only possible to achieve an increase in service life in the case of single homogenous ply media at the expense of the degree of separation, unless the installed filter is simply increased in size. This is not alone limited by increases in costs, but more especially also by limited space for installation so that for instance in the case of a pleated panel filter the number of the folds can not be increased to the necessary degree.
As a remedy the pleats in the impregnated paper in panel filters are presently covered on the inlet side with a foam material ply, which is to retain a fraction of the dust or at least reduce the kinetic energy of the particles so that there will be an increase in the service life. This method does however involve substantial disadvantages as regards production technology, since the layer of foam material must be bonded to the pleated panel after production of the panel in a further processing stage, for example using beads of hotmelt adhesive.
For internal combustion engines gradient filters are also employed, which are produced from synthetic fiber and become increasingly denser in the direction of flow through the filter. In this case the coarse particles are separated at the surface and the fines are deposited deeper in the filter. A disadvantage here is that for a given amount of installation space substantially fewer folds can be incorporated. This however increases the impact or inlet flow velocity with all the disadvantages connected therewith: higher pressure losses in the filter inherently owing to the higher flow velocity and deposit of the required dust quantity on less filter area so that the specific dust storage capacity must in this case be many times higher. Additionally such filter media make necessary a complete change in present day production systems, because sealing off the ends of the folds is no longer possible using conventional hotmelt technology. In fact, the bellow-like folds are injected directly in an injection molding method in a plastic frame in the case of such media, something which is comparatively involved.
Further present day methods for increasing the service life, for example for air conditioning and ventilation applications are described in the German patent publication 9,218,021.3 (utility model) or also the European patent publication 0 687 195. Here a fine filter layer of meltblown micro-fiber non-woven material, which determines the degree of separation, is covered with a coarse filter layer on the inlet side so that the dust holding capacity is boosted. The disadvantage is here that for a pleatable design a third layer is generally necessary, which provides the mechanical strength (more particularly stiffness) so that the pleated structure is self-supporting.
It is in the special case of cab air filtration for motor vehicles that melt blown non-woven materials are utilized as fine filters, whereas on the inlet side coarser structures, for example paper or spun non-woven material, serve as dust holding means.
The principle of melt blowing is described by Wente, Van A. in the article "Superfine Thermoplastic Fibers" in Industrial Engineering Chemistry, Vol. 48, pages 1342-1346. In gas or, respectively, air filtration generally such meltblown layers serve--owing to the fine fibers with a diameter or normally somewhat under 1 .mu.m to 10 .mu.m and owing to the frequently applied electret charge--as high efficiency separating filter layers and are for example described in the European patent publication 0 687 195, the German patent publication 9,218,021 (utility model) or the German patent publication 19,618,758, the fine meltblown layer always being employed of the outlet side (as a second filter layer). The support materials on the inlet flow side serve as dust storage means in the sense of deep filtration, the meltblown layer serves as a second filter stage in the sense of a fine dust filter. If a dust test is performed with the inlet flow on the "wrong side" that is to say with the meltblown side upstream, the initial degree of separation will be more or less identical, but the dust particle storage capacity goes down, i.e. an undesired filter cake is formed which increases the pressure loss on the inlet side with the meltblown layer surface.
The German patent publication 4,443,158 describes such a structure with the meltblown layer of the inlet side, the extremely high separating power of the meltblown material leading to a high degree of surface filtration, while the support material performs a purely mechanical function. The aim is however clear, i.e. to attain a good cleaning characteristic using for example compressed air after reaching the final differential pressure, but neither a high dust holding capacity during exposure to dust nor deep filtration. The meltblown layer here means an extremely high increase in the degree of separation, but simultaneously a reduction in service life in comparison with the second layer with extremely open pores.
One object of the invention is to provide a filter medium and an air filter with which the dust the holding capacity may be increased without any substantial change in the degree of separation and without any great increase in the thickness of the filter medium.